Measurement report: MAX-DOAS measurements characterise Central London ozone pollution episodes during 2022 heatwaves

Abstract

Heatwaves are a substantial health threat in the UK, exacerbated by co-occurrence of ozone pollution episodes. Here we report on the first use of retrieved vertical profiles of nitrogen dioxide (NO2) and formaldehyde (HCHO) over Central London from a newly installed multi-axis differential optical absorption spectroscopy (MAX-DOAS) instrument coincident with two of three heatwaves for the hottest summer on record. We evaluate space-based sensor observations routinely used to quantify temporal changes in air pollution and precursor emissions over London. Collocated daily mean tropospheric column densities from the high-spatial-resolution space-based TROPOspheric Monitoring Instrument (TROPOMI) and MAX-DOAS, after accounting for differences in vertical sensitivities, are temporally consistent for NO2 and HCHO (both R = 0.71). TROPOMI NO2 is 27 %–31 % less than MAX-DOAS NO2, as expected from horizontal dilution of NO2 by TROPOMI pixels in polluted cities. TROPOMI HCHO is 20 % more than MAX-DOAS HCHO, greater than differences in past validation studies but within the range of systematic errors in the MAX-DOAS retrieval. The MAX-DOAS near-surface (0–110 m) retrievals have similar day-to-day and hourly variability to the surface sites for comparison of NO2 (R ≥ 0.7) and for MAX-DOAS HCHO versus surface site isoprene (R ≥ 0.7) that oxidises to HCHO in prompt and high yields. Daytime ozone production, diagnosed with MAX-DOAS HCHO-to-NO2 tropospheric vertical column ratios, is mostly limited by availability of volatile organic compounds (VOCs), except on heatwave days. Temperature-dependent biogenic VOC emissions of isoprene increase exponentially, resulting in ozone concentrations that exceed the regulatory standard for ozone and cause non-compliance at urban background sites in Central London. Locations in Central London heavily influenced by traffic remain in compliance, but this is likely to change with stricter controls on vehicle emissions of NOx and higher likelihood of heatwave frequency, severity, and persistence due to anthropogenic climate change.